The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Wireless Worldwide Interoperability for Microwave Access (WiMAX) devices adhere to the I.E.E.E. 802.16 standard, which is incorporated herein by reference in its entirety. The WiMAX devices (hereinafter devices) include base stations (BSs) and mobile stations (MSs). A transmission from a BS to a MS is called downlink (DL), and a transmission from the MS to the BS is called uplink (UL). The BS and the MS may each include a single antenna or a plurality of antennas. The plurality of antennas is typically arranged in a multiple-input multiple-output (MIMO) configuration. Antennas arranged in the MIMO configuration (hereinafter MIMO antennas) provide better non-line-of-sight (NLOS) characteristics than single antennas.
Depending on the MIMO configuration used (i.e., depending on the number of transmit and receive antennas used), the devices may transmit data using different MIMO modes. For example, devices using two transmit antennas may transmit data mapped in a Matrix A mode and/or in a Matrix B mode. In the Matrix A mode, data symbols are mapped onto the transmit antennas using space time codes (STC). In the Matrix B mode, data symbols are mapped onto the transmit antennas using spatial multiplexing (SM).
In the Matrix A mode, two different data bit constellations are transmitted via two different antennas during the same symbol transmission. Conjugates of the same two constellations (e.g., the two constellations with sign changed) are transmitted again via the same antennas during another symbol transmission. In the Matrix B mode, a first data bit is transmitted via a first antenna, and a second data bit is transmitted via a second antenna during the same symbol transmission. The Matrix A mode offers greater range than the Matrix B mode. The Matrix B mode offers faster data rates than the Matrix A mode.
When transmitting data, the devices may select the Matrix A mode or the Matrix B mode depending on various factors. For example, the factors may include channel quality, number of receive antennas, quality of service requested, etc. Depending on changes in one or more of the factors, the devices may switch between the Matrix A mode and the Matrix B mode when transmitting data.
Occasionally, when a first device transmits data to a second device, the second device may not correctly receive the transmitted data. Accordingly, the first and second devices may use one or more techniques that enable the second device to detect and correct errors in the received data. The techniques comprise automatic repeat request (ARQ) and hybrid ARQ (HARQ).
When ARQ is used, the first device adds error detection (ED) bits (e.g., cyclic redundancy check (CRC) bits) to the transmitted data. When HARQ is used, the first device adds the ED bits and/or forward error correction (FEC) bits to the transmitted data. Typically, the ED and FEC bits are transmitted alternately on successive transmissions.
The second device transmits an acknowledgement (ACK) to the first device when the second device correctly receives the transmitted data. If the received data is erroneous, the second device corrects errors in the received data using the ED bits and/or the FEC bits and transmits the ACK to the first device. When the second device cannot correct the errors, the second device sends a retransmit request to the first device to retransmit the data. When the retransmit request is received, the first device retransmits the same data to the second device.
When the retransmitted data received by the second device has errors, the second device may attempt to correct the errors by combining the data received in the initial transmission and the data received in the retransmission. When HARQ is used, the second device may combine the data in two ways: chase combining and incremental redundancy. In chase combining, every retransmission contains the same information (data and parity bits). In incremental redundancy, every retransmission contains different information than a prior retransmission.